try this code:
here what i have done:
1. first you need to extract all files from archive to temp folder with given password.
2. then add files and set a new password again
3. delete the temp folder.
using (var zip = new ZipFile(@"C:\zip\Test.zip"))
foreach (ZipEntry entry in zip)
//Unzip all files with password to a temp
//Now zip with you new password
using (var zip = new ZipFile())
foreach (string file in Directory.GetFiles(@"C:\zip\Temp"))
ZipEntry entry = zip.AddFile(file);
entry.Password = "newpass";
please let me know in case of any concerns.